Make the bot play it first

A scripted player finds the broken numbers before a person has to. It can't tell you if it's fun.

Sapper's build 9 was tuned by three bots: a good player, a careless one and a rusher. They played whole runs headless, many seeds each, and the numbers changed from what they did: a cap on spitters, egg chambers in the hive, a queen with less health, cannons that hit harder.

The result was a sentence you can act on: the good bot reaches the hive on about a third of its runs, and no bot has beaten the queen yet. That's a difficulty curve you can see without asking anyone to play for an hour.

What a bot is good for

• Soft-locks and dead ends: a run that can't go on.
• Numbers that are off by a lot: a floor nobody survives, a gun nobody needs.
• Pacing: how long each step takes when it's played well.
• Checking a change didn't break the rest. Jumpgate has a bot that plays Chapter One from a new game and fails if it can't finish.

What it can't do

A bot plays the way you wrote it. In Swarmlight, two real sessions from the recorder showed the game was too easy: bots copied themselves zero or one times and nothing was ever lost. The bug was a copying radius that real play never reached, and the scripted players couldn't see it.

So the bot goes first, and people go second. And when a person finds something, change the bot so it plays a little more like them.

The cheap version

If your game keeps its state in one object, expose it on window (Sapper's is window.UP). Then a short script can step the game, read the state and print one line per floor. Ask your AI to write that script before you ask it for the next feature.